Polymer films or sheets, more particularly polyester films or sheets, especially those of polyethylene terephathalate (PET), are widely used in such applications as packaging, decorative articles, magnetic recording media, and photographic film. In such applications, the polyester film is printed or coated with a variety of materials, which must be applied as a uniform and continuous layer and must adhere well to the base polymer. However, PET and similar polymers do not have good receptivity to most surface-treating compositions. This problem has long been recognized in the prior art, and various methods have been developed to increase the polyester film's surface receptivity. A good discussion of the prior art in this area can be found in Posey et al., U.S. Pat. No. 4,476,189 issued Oct. 9, 1984. Nevertheless, most primers used commercially are oriented to specific end uses and are suitable only for either hydrophobic or hydrophilic surface treatment.
U.S. Pat No. 4,476,189 discloses a primed water-dispersible copolyester of terephthalic acid, an aliphatic dicarboxylic acid, and an aromatic sulfonated monomer with an alkylene glycol. The resultant primed polyester film is said to provide excellent adhesion to many subsequently applied aqueous or solvent-based coating.
The presence of the aromatic sulfonated monomer, which can be sodium sulfoterephathalic acid or a similar sulfonated derivative of a phthalic acid, is a shortcoming because such starting materials are not widely commercially available and therefore are expensive. Another potential shortcoming is that adhesion to the resultant primed polyester film can be humidity or moisture-sensitive. It is thus desirable to provide a primed polymer film or sheet which could be manufactured from inexpensive, readily available materials. It is further desireable to achieve a resultant primed polyester film to which adhesion is not humidity- or moisture-sensitive.